Optical recording and reproducing devices for recording and/or reproducing data with use of an optical recording medium by projecting a light beam thereto such as optical disk devices, optical card devices, and the like can perform high density recording since the diameter of a bit which is a recording unit of data can be easily arranged to about 1 .mu.m. Thanks to this advantage, the above type of recording and reproducing device has been in a spotlight in recent years as a recording device capable of recording a great amount of information.
In order to constantly perform such high density recording with a bit having a diameter of approx. 1 .mu.m, a tolerance on the order of a submicron is required for the diameter of the bit and focusing control and positioning control of the light beam or the like at the time of recording and reproducing of data.
Therefore, there is generally provided in an optical recording and reproducing device, a feedback control apparatus for executing a control loop which is composed of an actuator for controlling the illuminating conditions such as focusing and the illuminating position of a light beam projected onto a recording medium, a detection unit for detecting the aforementioned illuminating conditions, and a control amplifier for driving the actuator in accordance with an output signal from the detection unit.
Referring to FIG. 4, the control amplifier 10 which is a component of the feedback control apparatus comprises a subtraction circuit 11, a phase lead circuit 12, a phase lag circuit 13 and an amplification circuit 14. These circuits are connected in this order. In the substraction circuit 11, an output signal from the detection unit, according to the illuminating conditions of a light beam projected onto the recording medium, and an offset compensation signal are inputted to obtain the differential therebetween whereby a subtraction process is executed. In the phase lead circuit 12, a phase lag caused by the current-displacement conversion characteristic of the actuator in a high frequency region is electrically compensated so as to obtain a phase margin in the vicinity of the cut-off frequencies of the control loop, thereby preventing the control loop from being unstable.
The function of the phase lag circuit 13 is to increase the loop gain in a low frequency region, thereby improving the stiffness of the overall control loop and controlling errors in low frequency components caused by errors in initially setting the individual devices to be actuated by the actuator. The function of the amplification circuit 14 is to amplify the signal which has been compensated by the phase lead circuit 12 and the phase lag circuit 13 and to output the signal thus amplfied as a control signal for driving the actuator.
The signal level of a detection signal released from the detection unit becomes 0, for example, when the illuminating conditions of the light beam projected onto the recording medium are appropriate. Therefore, if a control amplifier having high accuracy in amplifying and compensating operation is adopted as the control amplifier 10, it will be ensured that the signal level of the control signal outputted from the control amplifier 10 is 0, thereby keeping the actuator in a stationary state.
In contrast, when an operational amplifier for general purpose is used as the operational amplifier comprising the phase lead circuit 12, the phase lag circuit 13, the amplification circuit 14 and the like, a DC offset is likely to be generated in the output signal from the control amplifier 10 due to a minute offset in the output of the operational amplifier caused by the characteristics of the operatational amplifier itself, slight fluctuation in the power voltage applied to the operational amplifier, etc. Such a DC offset tends to increase when increasing the overall gain of the control loop and the gain of the individual circuits which compose the control amplifier 10 to improve the responsiveness of the control loop and accuracy in the follow-up system thereof.
Therefore, even though the signal level of the detection signal outputted from the detection unit is 0, the signal level of the control signal from the control amplifier 10 does not necessarily become 0. This results in the flow of current into the actuator causing the actuator to be driven.
Positional errors X to be generated in the individual devices actuated by the actuator is approximately given by: EQU X=V.sub.of .times..DELTA.X/A
wherein
V.sub.of : DC offset in the control signal released from the control amplifier 10; PA0 .DELTA.X: Sensitivity of the actuator i.e., the ratio of the displacement of the individual devices actuated by the actuator to the voltage being applied to the actuator; PA0 A: Overall gain of the control loop.
The above positional errors X may prevent the illuminating conditions of the light beam projected onto the recording medium from being kept in an appropriate condition.
It is theoretically expected that the positional errors X will decrease when the overall gain of the control loop increases, since the positional error X is inversely proportional to the overall gain of the control loop. However, the positional error X practically increases since the DC offset increases in proportion to the increase in the overall gain of the control loop as mentioned above.
Further, when the DC offset increases, the output of the phase lag circuit 13 and amplification circuit 14 which have high-gain low frequency components approach the saturation level and these circuits do not properly work. As a result, it often occurs that the control loop is not normally executed and a control over the illuminating conditions of the optical beam projected onto the recording medium is entirely lost.
To solve the above problem, some conventional feedback apparatus are designed such that an offset compensation signal is entered in the subtraction circuit 11 to execute a differential input, the subtraction circuit being located before the phase lead circuit 12, phase lag circuit 13, and amplification circuit 14 which have high gain, and the signal level of the offset compensation signal is adjusted to a desired level by a variable resistor 15.
That is, in such apparatus, a DC offset caused by a minute offset in the output of the operational amplifier and slight fluctuation in the power voltage or the like is cancelled by the offset compensation signal. For example, when the signal level of the detection signal from the detection unit is 0, the signal level of the control signal released from the control amplifier 10 is controlled to be 0 thereby reducing positional errors.
Such a minute offset in the output signal of the operational amplifier and fluctuation in the power voltage, however, vary depending on operational amplifiers and power units. Therefore, conventionally, every feedback control apparatus in an optical recording and reproducing device is required to be adjusted in the manufacturing process in order to prevent the occurance of DC offsets. This additional adjusting process has brought about a high production cost.
Furthermore, if the power condition or the environment where the apparatus is used changes after adjustment has been done, a minute offset will be generated in the output signal of the operational amplifier and the power voltage will fluctuate. This makes it difficult to positively prevent the increase of DC offsets and an abnormal control operation.
It is naturally possible to employ an operational amplifier of high accuracy and a power unit having a high stability in order to prevent the occurrence of DC offsets, but this also brings about a considerable increase in the production cost.